1. Field of the Invention
This invention relates to a scale ideal for sensing the position and attitude of a moving object, as well as an apparatus for sensing a moving object by making use of this scale. More particularly, the invention relates to a sensing apparatus in which a scale is constituted by an angular grid which varies in two different directions in accordance with a known function, the angular grid being combined with an angle sensor, which moves relative to the surface of the angular grid, to thereby enable precise detection of the position and attitude of a moving object.
2. Description of the Related Art
When the position of a moving object such an XY table or cutter is sensed in a machine tool or the like, a measurement device such as a rotary encoder or linear encoder is necessary for each degree of freedom. When two-dimensional positioning is performed, for example, stages capable of being positioned along respective ones of x and y axes are stacked one above the other and a measurement device is provided for each stage to achieve positioning. Alternatively, a measurement device comprising a combination of a circumferential scale and a single-axis stage is used to measure rotational position and radial position independently to effect positioning.
In a situation where position is determined in x and y directions using a laser interference-type displacement meter, position is sensed by a combination of two displacement meters and a device such as a highly precise straight edge the accuracy of the shape whereof is assured over a range of movement at right angles to the direction in which displacement is sensed.
Further, in cases where attitude corresponding to pitching and yawing of a moving object is sensed in the prior art, use is made of an autocollimator. Though an autocollimator is capable of measuring pitching and yawing simultaneously with respect to linear movement along one axis, a high-precision straight edge is needed for detection of an object moving along the two x and y axes.
Furthermore, though a leveling instrument is known as means for measuring the rolling of a moving object, problems arise in terms of speed of response and measurement accuracy and therefore a leveling instrument is not suitable for use as highly precise measurement equipment.
Accordingly, the state of the art is such that two parallel straight edges are deployed and rolling angle is calculated from the difference between distances measured up to the straight edges, or such that rolling angle is sensed by an autocollimator using a single straight edge as a reference mirror surface.
However, a measurement device such as the rotary encoder or linear encoder used in the conventional sensing apparatus described above is capable of performing positioning in one dimension only. At least two sets of the above-mentioned measurement apparatus are required to be combined for two-dimensional positioning, a fact that represents a major limitation when designing an apparatus for sensing moving objects.
Further, in a situation where positioning is performed using the laser interference-type displacement meter, essentially only one-dimensional positioning is possible. If positioning in two dimensions is to be carried out, the highly precise straight edge is required. As a result, in a scenario where a machine tool or the like is provided with this type of apparatus for sensing moving objects, problems arise in terms of manufacturing limitations and higher cost.
With the prior-art sensing apparatus, the encoder which determines position and the measurement circuitry which senses a change in attitude are separately arranged. Consequently, a sensing apparatus capable of sensing the two-dimensional position, pitching, rolling and yawing of a moving object would be complex and high in cost.
With means such a photoelectric linear scale, a high degree of positional accuracy is required to correctly place the scale and the device that reads it. This makes it difficult to deploy a plurality of reading devices in an effort to enlarge the range of measurement beyond the length of the scale. The end result is that a long scale is required in order to sense movement along a long range of movement.